The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Data communication rates are ever increasing. This is especially true with respect to serializer/deserializers (SERDES). In general, with increased communication speeds comes increased inter-symbol interference (ISI). ISI is a function of a communication channel through which a data signal passes and a swing voltage of that signal. ISI changes voltage and timing components of a transmitted signal. To cancel the introduced ISI, equalization is implemented to provide reliable and interpretable data.
Among available equalization methods, receive side adaptive equalization, such as feedforward equalization and decision feedback equalization, are typically used. Adaptive equalization does not require advanced knowledge of channel information or an additional feedback channel from the receiver to the transmitter.
The use of receive side adaptive equalization introduces difficulty in physically measuring a characteristic of an “eye” that is associated with a signal at a decision point of a circuit. When the traces of a signal are overlaid a signal pattern is generated, which may be viewed using a lab instrument, such as an oscilloscope or other eye diagram analyzer. When the data of the signal is not in the form of a simple repeating pattern, the overlay of the traces forms a shape that resembles a partially closed eye. The decision point of a circuit refers to a data recovery point of that circuit, or in other words, the point after signal conditioning and prior to data recovery. The conditioning circuitry can affect signal shape and other signal aspects, such as jitter and amplitude. The decision point is sometimes referred to as the point at which the data is digitized. The characteristics of the eye, such as the eye opening, eye width, eye amplitude, etc., are observed to evaluate the received data signal. In general, the larger the open or clear area within the eye and/or the less traces within the eye, the better the data signal.
The characteristics of the eye can be evaluated. The eye characteristics provide information regarding the quality of the signal received by a digital sampler or data recovery circuit. The eye characteristics also provide information regarding the performance of signal conditioning circuitry and the types of signals a data recovery circuit can handle.
Using a lab instrument to measure and evaluate the eye of a signal at a decision point, in high speed communication applications, is difficult and the results from which are often inaccurate. The electrical connections that are used between the lab instrument and the decision point of the circuit under test, such as the wiring, bond connections, pins, pads, etc., causes the data signal to distort significantly. Thus, for example, a measured eye opening may not be the true eye opening at the decision point. Also, eye measurements can not be made within an integrated circuit chip via a lab instrument and cables.